1. Field of the Invention
The present invention generally relates to a communication system and a communication method, in which a plurality of wireless communication stations severally operates as if each of them is controlled under self-organized distributed control to form a wireless network including a plurality of communication channels for performing data communication among the plurality of wireless communication stations, a communication apparatus, a communication control method and a computer program to be used in the communication system and the communication method. In particular, the present invention relates to a communication system, a communication method, a communication apparatus, a communication control method and a computer program, all can be suitably applied to the so-called wireless local area network (WLAN).
2. Description of Related Art
Wireless communication techniques for connecting various information processing stations, such as personal computers and personal digital assistants, and their peripheral equipment wirelessly have recently been developed. As a representative one of the techniques, the wireless LAN complying with the so-called Institute of Electrical and Electronics Engineers (IEEE) 802.11 system has been spreading.
In the wireless LAN of the IEEE 802.11 system, contention-free periods for performing media access control (MAC) by polling and contention periods for performing the media access control by carrier sensing are standardized as a technique of the media access control system pertaining to a protocol of distributed control, centralized control or the like of a data link layer. The contention periods for performing the carrier sensing are widely used.
In specific, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) system following the self-organized distributed control system used in the so-called Ethernet (registered trademark) is standardized as the contention periods. The CSMA/CA system is a system roughly described as follows. That is, a communication station trying to transmit data performs carrier sensing in advance, lest the data should collide with data transmitted by another communication station, to confirm the status of a communication channel to be used. When a band of the communication channel is unused, the former communication station transmits the data. When the band is in use, the communication station postpones the transmission of the data until the band becomes an idle state. The wireless LAN of the IEEE 802.11 system provides an infrastructure mode and an ad hoc mode. In the infrastructure mode, there exists an access point (AP) as a control station and a plurality of stations (STA's) existing in a range where a radio wave of the access point (AP) is reachable are under control of the access point. In the ad hoc mode, there exists no access point and a plurality of stations operates as if each of them is controlled under self-organized distributed control as the methods of networking based on the concept of the so-called basic service set (BSS). Both of the access point and the plurality of stations perform communication by following the processes complying with the CSMA/CA system.
To put it concretely, the communication operation performed in the wireless LAN of the IEEE 802.11 system will be described below.
First, the operation in the infrastructure mode will be described.
In the BSS of the infrastructure mode, an access point for performing coordination is essential in a system. In the infrastructure mode, for example as shown in FIG. 19, a range of a radio wave of an access point STA0 is collected as a BSS to constitute an area corresponding to a cell in the so-called cellular system. Consequently, stations STA1 and STA2 included in the range of the radio wave of the access point STA0 join a network as a member of the BSS. Then, the access point STA0 transmits control signals called “beacons” at predetermined time intervals. Accordingly, each of the stations STA1 and STA2 receives the beacon signals, and thereby recognizes the access point STA0 existing in the neighborhood. Then, each of the stations STA1 and STA2 performs establishment of a connection to the access point STA0.
As described above, the access point STA0 transmits the beacon signals at the predetermined time intervals. The next transmission time of one of the beacon signals is determined by a parameter described in the beacon signal. The parameter is called a target beacon transmission time (TBTT). Each of the stations STA1 and STA2 receives the beacon signals and refers to the TBTT. Thereby, the stations STA1 and STA2 can grasp the next transmission time of the beacon signal. Consequently, for example, in a case where there are no needs of receiving the beacon signals, each of the stations STA1 and STA2 can also shut off the power source of its receiver for the next TBTT or for a plurality of TBTT's hence to shift to a sleep mode.
Next, the operation in the ad hoc mode will be described.
In the ad hoc mode, each station self-organizedly defines an independent BSS (IBSS) by performing predetermined negotiations with the other stations. In the ad hoc mode, for example, as shown in FIG. 20, when the IBSS has been defined, each of the stations STA1 and STA2 determines a TBTT after the negotiations at every passage of the predetermined time interval. Then, when each of the stations STA1 and STA2 recognizes the arrival of one of the TBTT's by referring to a clock equipped in each of the stations STA1 and STA2, and when each of the stations STA1 and STA2 recognizes that no other stations transmit any beacon signals after a retardation for a period of time determined by uniform random numbers, each of the stations STA1 and STA2 transmits a beacon signal.
Incidentally, also in the IBSS, each of the stations STA1 and STA2 can shut off the power source of its transmitter/receiver for the next TBTT or for a plurality of TBTT's hence to shift to a sleep mode as the need arises. To a case where a sleep mode is applied in an IBSS, the IEEE 802.11 system defines a predetermined time zone from a TBTT as an announcement traffic indication message (ATIM) window.
All of the stations belonging to an IBSS operate their receivers in the time zone of an ATIM window. In this time zone, it is basically made to be possible to perform a reception even when the stations operate in their sleep modes. When each station holds data which should be transmitted to an arbitrary station, the station informs the arbitrary station of the holding of the data to be transmitted by transmitting an ATIM packet to the arbitrary station after the transmission of a beacon signal in the ATIM window time zone. Then, the station which has received the ATIM packet operates its receiver until the station has completed the reception from the station which has transmitted the ATIM packet.
Now, for describing the operation concretely, for example, as shown in FIG. 21, a case where three stations STA1, STA2 and STA3 are present in the IBSS is explained. In this case, each of the stations STA1, STA2 and STA3 watches the states of media over a period of time determined by uniform random numbers while operating a timer of a backoff provided for the avoidance of collisions of data after the time of TBTT has elapsed. Hereupon, a case where the timer of the station STA1 has completed its count fastest and the station STA1 has transmitted a beacon signal is shown. In this case, because the other stations STA2 and STA3 have recognized the transmission of the beacon signal by the station STA1, the stations STA2 and STA3 do not transmit any beacon signals.
Moreover, it is supposed that the station STA1 holds data addressed to the station STA2 and the station STA2 holds data addressed to the station STA3. First, when the stations STA1 and STA2 have completed the transmission and the reception of beacon signals, the stations STA1 and STA2 again operate the timers of backoff while watching the states of media over periods of time determined by uniform random numbers. Hereupon, when it is supposed that the timer of the station STA2 has completed its count fastest, the station STA2 transmits an ATIM packet to the station STA3. In response to the transmission of the ATIM packet, the station STA3 transmits the so-called acknowledgement (ACK) signal after a predetermined period of time, or a short inter frame space (SIFS), from the reception of the ATIM packet. Then, when the transmission of the ACK signal from the station STA3 has been completed, the station STA1 watches the states of media over a period of time determined by uniform random numbers while operating the backoff timer. When the period of time set in the timer has elapsed, the station STAL transmits an ATIM packet to the station STA2. In response to the transmitted ATIM packet, the station STA2 transmits an ACK signal after a predetermined period of time SIFS from the reception of the ATIM packet.
In the wireless LAN of the IEEE 802.11 system, when the transmissions and the receptions of such an ATIM packet and an ACK packet have been performed in the time zone of the ATIM window, the station STA3 operates the receiver for receiving data from the station STA2, and the station STA2 operates the receiver for receiving data from the station STA1 even in the intervals after the transmissions and the receptions. Then, in the wireless LAN of the IEEE 802.11, the stations which have not received any ATIM packets in the time zone of the ATIM window and the stations which do not hold any data which should be transmitted can shut off the power sources of the transmitter/receivers until the next TBTT to achieve reduction of their electric power consumption.
Incidentally, as the technique pertaining to the access control at the time of such wireless communication, for example, a technique described in Patent Document 1 is noted. Patent Document 1 discloses the technique for a cordless handset to detect the existence of the radio waves of control signals from other base phones at an interval of the transmission timing of control signals from a base phone in response to an inquiry of an radio wave situation by the base phone, and to report the existence information of the radio waves to the base phone. Then, Patent Document 1 disclose a technique such that the base phone detects time difference between the transmission timing of another base phone and the transmission timing of the own station when a radio wave of a control signal of the other base phone is present, and that the base phone changes the transmission timing of the control signal of the base phone when the detected time difference is equal to or less than a previously determined value.
[Patent Document 1] Japanese Patent Application Publication Hei 8-217914
Now, the wireless LAN of the IEEE 802.11 system uses the so-called unlicensed spectrum (bandwidth) such as 5.2 GHz band and 2.4 GHz band. Those bands are further divided into a plurality of bands. Each communication station performs communication using one of the divided bands. For example, in the IEEE 802.11a, the band of 5.2 GHz is divided into four bands by the 20 MHz. A station trying to perform wireless communication selects one of the divided bands as its communication channel.
Hereupon, in the wireless LAN of the IEEE 802.11 system, as described above, communication is started after confirmation of unoccupied communication channels in accordance with processes complying with the CSMA system. Consequently, it is necessary for the stations performing the wireless communication to use the same communication channel. Hence, in the wireless LAN, though the band is divided and a plurality of communication channels are prepared, the communication channel which is actually used for transmission of signals is substantially only one channel.
Moreover, because those bands are unlicensed spectrum, there can be a case where wireless LAN's of systems different from each other use the same communication channel. In such a case, interference from another system can be avoided by changing the communication channel of the own system. However, it is necessary for realizing the avoidance to detect the presence of the other systems.
Incidentally, as a technique for selecting an unoccupied communication channel for avoiding the interference with the other systems in the situation in which a plurality of systems are present, there is a technique described in, for example, Patent Document 2.
[Patent Document 2] Japanese Patent Application Publication No. 2002-158667
Patent Document 2 discloses a technique for selecting an optimum channel to the changes of the neighboring situation by judging the radio wave situations of all of the wireless channels capable of being used by a control station at its rising to determine the optimum channel, and by stopping the transmission of the own network periodically to watch the neighbor radio wave situation by dispersed mobile terminals.
Now, as a wireless communication system different from the wireless LAN, there is a portable cellular phone system called as the so-called personal digital cellular (PDC). In such a portable cellular phone system, a control station manages the control information of stations in a concentrated manner, and controls the communication time and the communication channel of each station.
In a system adopting a centralized control system such as the portable cellular phone system and the infrastructure mode in the wireless LAN of the IEEE 802.11 system, a control station or an access point which performs the centralized control bears a great burden. Moreover, the system becomes large in scale, and the costs for constructing the system increases. Moreover, the system adopting such a centralized control system has a problem of the difficulty of the construction of a network.
On the other hand, in a system adopting a self-organized distributed control system such as the ad hoc mode in the wireless LAN of the IEEE 802.11 system, each station independently judges the neighboring situation, and a network can be easily expanded.
However, the conventional techniques pertaining to the selection of a communication channel such as the technique described in Patent Document 2 and so forth are supposed to be applied to the centralized control system using a control station, an access point and the like, and such conventional techniques cannot be applied to the systems adopting the self-organized distributed control system.
As mentioned above, the system adopting the self-organized distributed control system cannot use an access control system by carrier sensing like the CSMA system while using a plurality of communication channels simultaneously. Consequently, the system adopting the self-organized distributed control system inevitably has a problem of limitations of channel capacities (communication capacities).